**3.5 Synthesis of gold nanoparticles**

The method presented by Abdollahi et al. [10] was followed to elaborate the GNPs. First, 100 ml of deionized water at room temperature was placed in a 140 ml flask under magnetic agitation. Then 1 ml of 1% HAuCl4 solution, 2 ml of 1% trisodium citrate, and 1 ml of freshly made 0.075% NaBH4 in 1% trisodium citrate were added in that order. The mixture was stirred for 10 min and used immediately to avoid the agglomeration. The GNP may also be stored at 4°C in an amber glass bottle for later use.

Throughout the synthesis, the gold solution changed its color from light yellow (**Figure 2a**) to wine red (**Figure 2b**). This is a characteristic of the GNP formation [26].
